Periodontal disease is a chronic bacterial infection that affects the gums and bone supporting the teeth. Periodontal disease begins when the bacteria in plaque (the sticky biofilm that constantly forms on teeth) causes the gums to become inflamed. Periodontal disease can affect the gingival tissue (gums); periodontal membrane (connective tissue embedded in the cementum and alveolar bone); cementum (mineralized connective tissue covering the roots of the teeth); and the alveolar bone (bone socket). Depending on the progression of the disease, there may occur a destruction of periodontal membranes, alveolar bone loss, and apical migration of the connective tissue attachment. Advanced periodontal disease may result in the formation of periodontal pockets harbouring bacterial plaque, and progressive loosening and eventual loss of teeth. Periodontal disease includes gingivitis that can advance to periodontitis. Chronic periodontitis is an inflammatory disease of the supporting tissues of teeth that is associated with specific bacteria in subgingival dental plaque. The disease has been estimated to affect around 35% of dentate adults and is a major cause of tooth loss in the Western world(1). P. gingivalis, a member of the normal oral microflora of subgingival dental plague, has been implicated as one of the major opportunistic pathogens in the progression of this disease(2).
P. gingivalis is a black-pigmented, asaccharolytic, Gram-negative anaerobic, cocco-bacillus, that relies on the fermentation of amino acids for energy production(3). Like most bacteria, P. gingivalis has an essential growth requirement for iron that it preferentially acquires in the form of haem, a molecule comprised of a protoporphyrin IX ring (PPIX) with a co-ordinated central ferrous atom(4). This utilization of haem as an iron source may reflect the inability of P. gingivalis to synthesize PPIX de novo(5). Haem is preferentially obtained from haemoglobin, and is acquired through the activity of the cell-surface Arg- and Lys-specific proteinase/adhesin complex(4,6,7), possibly in conjunction with a TonB-linked outer membrane receptor, HmuR(5). Unlike aerobic or facultative bacteria that obtain iron using siderophores P. gingivalis does not produce siderophores and lacks the ferric reductase activity usually associated with siderophore-mediated iron acquisition(9,10). P. gingivalis stores haem on its surface in the form of μ-oxo bis-haem, which has inherent catalase activity that helps to protect the cell from oxidative attack(11). For P. gingivalis to be able to compete with the large numbers and diversity of bacteria within the micronutrient-limiting environment of the oral cavity(12) it not only has to establish itself but also has to evade or overcome numerous host defences.
The initiation and progression of periodontal disease is associated with bleeding at the site of disease, thereby providing an elevated level of haemoglobin. Therefore in order to help understand the mechanism by which P. gingivalis establishes and proliferates in subgingival plaque and initiates disease it is important to determine the changes in relative protein abundances of P. gingivalis during the transition from micronutrient poor (haem-limitation) to rich (haem-excess) conditions.
Although many proteins have been associated with growth under haem-limitation(9,10), no extensive work on the P. gingivalis proteome or the changes to the proteome during haem-limitation has been reported.
In developing compositions which would be useful in the prevention and treatment of periodontal disease it is desirable to identify agents that interfere and prevent the initial stages of the disease process.
The present inventors have now developed methods for identifying specific P. gingivalis proteins regulated by haem availability that can be used as suitable targets for the prevention and treatment of periodontal disease.